The present invention relates generally to electronic music systems and more particularly to an electronic music system which generates an output signal representative of the pitch of a musical signal.
Musical signals are vocal, instrumental or mechanical sounds having rhythm, melody or harmony. Electronic music systems employing a computer which receives and processes musical sounds are known. Such electronic music systems produce outputs for assisting a musician in learning to play and/or practicing a musical instrument. Typically, the computer may generate audio and/or video outputs for such learning or practicing representing a note, scale, chord or composition to be played by the user and also, audio/video outputs representing what was actually played by the user. The output of the electronic music system which is typically desired is the perceived audio frequency or "pitch" of each note played, provided to the user in real time, or in non-real time when the musical signal has been previously recorded in a soundfile.
Certain electronic music systems rely on keyboards which actuate switch closures to generate signals representing the pitch information. In such systems, the input device is not in fact a traditional musical instrument. Desirably, an electronic music system operates with traditional music instruments by employing an acoustic to electrical transducer such as a magnetic pickup similar to that disclosed in U.S. Pat. No. 5,270,475 or a conventional microphone, for providing musical information to the electronic music system. Such transducers provide an audio signal from which pitch information can be detected. However, due to the complexity of an audio signal waveform having musical properties, time domain processing, which relies principally on zero crossing and/or peak picking techniques, has been largely unsuccessful in providing faithful pitch information. Sophisticated frequency domain signal processing techniques such as the fast Fourier transform employing digital signal processing have been found necessary to provide the pitch information with the required accuracy. Such frequency domain signal processing techniques have required special purpose computers to perform pitch detection calculations in real time.
One problem faced in detecting pitch from a musical signal is that caused by noise within the signal, i.e. that part of the audio signal that can not be characterized as either periodic or as the sum of periodic signals. Noise can be introduced into the musical signal by pickup from environmental sources such as background noise, vibration etc. In addition, noisy passages can occur as an inherent part of the musical signal, especially in certain vocal consonant sounds or in instrumental attack transients. Such noise adds to the computational burden of the pitch detection process and if not distinguished from the periodic portion of the signal, can bias the pitch measurement result. Traditional methods for removing noise using frequency domain filtering the signal are only partially successful because the noise and the periodic portion of the signal often share the same frequency spectrum. Alternatively, the noise passages may be excised. Conventionally, the noisy passages to be excised are identified by autocorrelating the musical signal. However, the autocorrelation technique has proven unreliable in distinguishing noise from the complex periodic waveforms characteristic of music.
A problem one faces when using frequency domain signal processing techniques is the introduction of artifacts into the output of the analysis. Such artifacts are introduced when the frequencies present in the musical signal to be processed are not harmonically related to the digital signal processing sampling rate of the musical signal. The Fourier transform of such sampled signals indicates energy at frequencies other than the true harmonics of the fundamental frequency, leading to inaccurate determination of the pitch frequency.
The present invention provides an improved method for detecting the pitch of instrumental, vocal and other musical signals, such method reducing the computational burden of pitch detection sufficiently to allow real time pitch detection to be implemented on a standard personal computer having a standard sound input card, without the need for additional hardware components. The present invention overcomes the problems introduced by noise by providing a computationally efficient noise detection method. The noise detection method, based on computing the local fractal dimension of the musical input signal, provides a reliable indication of noise for removing noisy time segments of the signal from the processing stream prior to measuring the pitch. The present invention further provides an improved spectral analysis method, based on multitaper spectral analysis, which reduces the magnitude of artifacts in the spectral analysis output, thus improving the accuracy of the pitch measurement. By the simple addition of driver software to a standard personal computer and connection of an audio transducer or a microphone into the sound card input port, a user is able to observe an accurate, real time rendition of an acoustic, wind or percussion instrument, the human voice or other musical signals and to transmit the information representing the rendition over a computer interface to other musicians, educators and artists.